System for reproducing sounds from distorted sound records



May 21, 1929. A. POULSEN HAL 1,713,918

SYSTEM FOR REPRODUCING SOUNDS FROM DISTORTED SOUND RECORDS Filed Nov. 19, 1927 /4/"7202d 2,025 and 42d Car Geo/:9 Flerser Attorney Patented May 21, 1929.

UNITED STATES PATENT OFFICE.

ARNOLD POULSEN AND AXEL CARL GEORG PETERSEN, OF COPENHAGEN, DENMARK.

SYSTEM FOR REPRODUOING SOUNDS FROM DISTORTED SOUND RECORDS.

Application filed November 19, 1927. Serial .No. 234,498.

It iswell known that in the electrical transmission of sounds or music, either through wires or wirelessly, certain difliculties arise due to the fact that tones of low frequencies, which impart t0 the human ear impressions of the same power as impressions imparted to the ear by tones of high frequencies, have amplitudes which are several times greater than the amplitudes of the said tones of high frequencies.

If sound and music were transmitted without distortion, the fact referred to above would necessitate that the dimensions of the transmission means in question-for instance, the amplifiers, wireless transmitters etc-having essentially the same sensitiveness to hi her and lower frequencies, be determined y the powerful influence of tones of low frequencies, in which case the transmission means would be very expensive. Furthermore, when recording phonograms the low frequencies would dominate the mean and high frequencies, so that when the low frequencies would attain the maximum amplitude available the mean and the high frequencies would be so very diminutive in the phonogram that the size of thegrains in the sensitizin emulsion onthe film and of the unavoida le defects in the film itself might be considerably relatively to the lastnamed variations and, consequently, cause distortions in the reproduction of the sounds.

. In order to avoid these difficulties, we have attempted to weaken by artificial means the amplitudes of low frequencies during the recording in such a way that sound variations with one and the same intensity .of sound to the human ear independently of their frequency will produce current variations of practically one and the same ampli- 1 tude; and to annihilate this weakening during the reproducing by using a member which has a greater sensitiveness to the low frequencies than to the highfrequencies.

Now, we have found that this compensation may beefiected to a certain'degree by using aloud speaker with a loosely suspended] or floating diaphragm, such loud speakers having the characteristic property of preferring the very low frequencies. However, it has been found that the compensation which may be obtained in this way is, in the first place, insufiicient and, in the second place, does not allow any fine regulation of the proportion between the low and high frequencies which may be necessary in order to obtain a complete compensation.

To overcome these difliculties, we utilize the well-known phenomenon that the sensi-v tiveness of a selenium cell increases when the frequency of the light variations influencing the cell decreases. g

The reproducing system includes in wellknown manner an inductance and an ohmic resistance, which are interconnected in series and are connected across the id circuit of a thermionic tube, in the grid circuit of which a selenium cell is inserted. According to the invention, the value of the resistance is determined in relation to the value of the inductance in the reproducing system and of the capacity and the ohmicresistance in the recording system as referred to hereinafter.

A. more thorough understanding of our invention may be had from the following detailed description taken in connection with the accompanying drawing, in which: I

Figure 1 illustrates an ordinary sound recording. system, wherein the sound amplitudes of low fre uencies are weakened during the recording; producing system and Fig.3 is a diagram illustrating the distortion caused duringthe recording and the counter-distortion caused during the reproducing.

The sounds to be recorded on a film f are caused to influence a transmitter, preferably 'a condenser microphone 0,-Fig. 1, which is inserted in circuit with the grid 9 of a thermionic tube't, such circuit also containing a batter b, a resistance R, a grid resistance 3 and a blocking condenser k. Now, by choosing the value of the resistance R properly in ig. 2 illustrates a sound re 7 relation to the capacity of the condenser microphone c, it is possible to obtain that sound variations with one and the same intensity of sound to the human ear. independently of their frequency will produce current variations of practically oneand the same amplitude in the anode circuit a of the tube t, wherefore a registering device, for instance an oscillograph m, which is influenced by said current variations and reflects a beam of light from a light source Ztowards the film f, will record a distorted phonogram p on the latter.

If the condenser microphone c is influenced by a sound variation with the frequency w=21r.n, then the said condenser will act as a generator. If the electromotive force thereof is E, and the generator, i. e., the microphone 0, is coupled to a resistance B (Fig. 1) having a small. value relatively to the grid resistance y, then the voltage variation superimposed on the grid is expressed by where C is the capacity of the condenser microphone c.

1 The distortion d of the sound amplitudes in the phonogram p or, more exactly, of the current variations caused in the anode circuit a of the recording system, varies in harmony with the expresslon and is illustrated by the curve I in Fig. 3.

If, in the reproducing system llustrated in Fig. 2, the film f is moved longitudinally and light is passed through the film and concentrated on a selenium cell 8, by means of a lens 2', which forms on the cell an image of the phonogram p, then the electrical conductivity of the selenium cell 8 will vary, and these variations are proportional to the amplitudes in the phonogram p and at the same time inversely proportional to the frequencies of the light variations.

If the selenium cell 8 is inserted in the rid circuit of a thermionic tube t, and an in uctance L is connected across said circuit, then the inductance L will compensate the distortion, which the well known sluggishness of the selenium cell causes in the current amplitudes of the reproducing system.

Now the phonogram p is a distorted sound record and, therefore, a sound reproducer, for instance a loud speaker k, in the anode circuit A of the tube T would reproduce distorted sounds unless the distortion caused during the recording of the sound is annihicounter-distortion is caused partly by the use of a loud speaker of the floating-diaphragm type and partly by the insertion of an ohmic resistance 1' in series with the inductance L. The said resistance 7' will prevent, by the low frequencies, the distortion, which is due to the sluggishness of the selenium cell, from being compensated by the inductance L and, therefore, will influence the current amplitudes in the reproducingcircuits; and if the resistance r is properly detcrn'iiued, it is possible to obtain a complete compensation of the distortion introduced in recording.

If, for a moment, we neglect the compensating effect which may be obtained by using a loud speaker with floating diaphragm, we can show that a complete counter-distortion may be effected by properly determining the resistance 1'.

If the variation caused in the voltage across the selenium cell 8 by the illumination of the same, is E, then said voltage variation is expressed by where K is a value which varies proportionately to the amplitudes in the phonogram.

If the grid resistance a: of the tube T is, large relatively to the impedance of the inductance L and of the resistance 7', then the voltage variation superimposed on the grid of the tube T is where p is the ohmic resistance of the selenium cell 8. If

by the highest value of the frequency w is small relatively to p, then the expression for A 0 may be reduced to or by the insertion of g for E to The amplitudes of the reproduced sounds vary proportionately to A e, and because A 6 depends upon the frequency and increases when the frequency decreases, the reproduced sounds are distorted in comparison with the distorted sound amplitudes in the phonogram. The said distortion caused in the reproducing system is illustrated by the curve II in Fig. 3, from which it is evident that the distortion in the recording system is counteracted by the distortion in the reproducing system.

The first distortion, expressed by the value a (R- W (see above) will'be' annihilated by the second distortion expressed by the value or consequently if r 1712-0 *R-U From this formula, it is evident that the distortion of the low frequencies during the recording may be entirely compensated in the reproducing systemby proper selection of the value of the ohmic resistance 1'.

We claim as our invention 1. A system for reproducing sounds from phonograms, the amplitudes of which are distorted relatively to the amplitudes of the original sounds during their previous conversion into current variations in such a way that the lower frequencies are suppressed in relation to the higher frequencies: such reproducing system comprisin a selenium cell adapted to be illuminated by light passing through the phonogram; an inductance; a source of current; a current circuit wherein said cell, inductance and current source are included in series; a thermionic tube having a grid circuit and a plate circuit, the grid circuit being connected across the inductance; a sound reproducer inserted in the plate circuit; and means for annihilatcuit across which the resistance is connected and a plate circuit, and a sound recorder inserted in the plate circuit: such reproducing 'system comprising a selenium cell adapted to be illuminated by light passing through the phonogram; an inductance; a

resistance; a source of current; a current cir-' cuit wherein said cell, inductance, resistance and current source are included in series; a thermionic tube having a grid 'circiut and a plate circuit, the grid circuit being connected across the inductance and the resistance in series; and a sound producer inserted in the plate circuit; the said resistance having the value where L is the value of the inductance in the reproducing system, and R and C the values of the ohmic resistance and the capacity, respectively, in the recording-system.

In testimony whereof we afiix our signatures.'

ARNOLD PonL'snN. AXEL CARL GEORG PETERSEN. A 

